This invention relates to low density building products and methods for their production.
Shaped calcium silicate products are widely used in industry and domestic applications as thermal insulation and refractory materials and shown in Australian Patent No 552,930. These products are commonly formed by combining a source of calcium hydroxide with a siliceous material at elevated temperature and pressure. A variety of crystalline forms of calcium silicate may be formed depending upon the temperature, pressure, length of reaction time and water concentration used. The resultant slurry of hydrated calcium silicate crystals may then be cast into moulds and dried, usually by the application of heat, to form the desired finish shaped products prior to moulding if desired. Reinforcing fibres such as glass and polyester fibres may then be added to the slurry.
Such pure calcium silicate products, however, do not have sufficient strength, toughness and durability for various building applications. Various mechanisms have been proposed for producing calcium silicate products with sufficient strength for building applications. Low density calcium silicate boards using fibre contents of 3% by weight or less are well known in fire protection applications. The method of manufacture of the calcium silicate in these products and the other ingredients needed for high temperature ( greater than 1000xc2x0 C.) stability make such product uneconomical for use as commodity products. For example, U.S. Pat. No. 5,547,505 discloses a production method for low heat cements with good compressive strength and heats of hydration. The process is, however, quite complex. It involves the mixing of calcareous materials, siliceous materials and aluminous materials, melting the mixture, quenching the melt and grinding the quenched matter for inclusion in the cement composition.
Australian Patent No 606,344 attempts to overcome the difficulties associated with calcium silicate products by adding fibrous materials, such as wood pulp, and a suitable polymeric coagulant, forming thin sheets of the resulting material and laminating the sheets to form a building board. Various other production techniques or additives such as fibres, polymeric binders etc have been used to increase the strength and durability of the calcium silicate product. German Patent Application DE 3711549 for example includes a polymeric binder and an expansion ettringite forming agent to the calcium silicate slurry to give a product similar to wood.
Fibre reinforced cement products such as water resistant building sheets have been used for building since 1895. In recent history reinforcing fibres used in such products have included not only asbestos fibres, but also cellulose fibres (AU 515151), metal fibres, glass fibres and other natural fibres. Typically the density of such building sheets is from 1.2-1.7 g/cm3, the variation in density typically being achievable by compression and dewatering of the fibre cement slurries used in manufacture and by varying the amount of fibre used.
The densities of the building sheets described above mean the products are heavier than timber based products of equal dimension and consequently are more difficult to cut, machine and nail than timber and timber based products. In this regard, the density of natural timber sheets typically range from 0.7 to 0.9 g/cm3 for dry hardwoods and from 0.38 to 0.6 g/cm3 for dry softwoods. The dry density of reconstituted pressed hardboards typically range from 0.95 to 1.1 g/cm3.
It would be desirable to manufacture building sheets having a density similar to that of timber thus enabling lighter, more nailable, easier to cut and easier to machine products whilst retaining the durability, fire resistance, rot proofness and water resistant properties of the fibre reinforced cement product.
It is an object of the present invention to substantially ameliorate at least some of the above-mentioned disadvantages of the prior art or at least provide a commercial alternative thereto.